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by Martin Khor
The Nipah virus
that has apparently spread from pigs to humans highlights the increasing
evidence that disease-causing microbes are breaking species barriers as
they cross over from animals to people. This process is now thought to be
caused by horizontal gene transfer which must be more properly understood
if we are to prevent the outbreak of more new diseases.
It is now confirmed that the epidemic causing brain damage and death in
Negri Sembilan and Perak is mainly caused by a newly discovered virus,
named the Nipah virus.
This has similarities with the Hendra virus and distinctly different from
the Japanese encephelatis virus. Whilst the latter is spread via the culex
mosquito, it is not clear how the Nipah virus is spread.
What seems certain is that the Nipah virus is transferred from pigs (some
of which have in the affected areas also been afflicted with a similar
disease) to people who have been involved in handling pigs.
The virus attacks the brain, causing many of the victims to go into a coma
and some to die.
In Australia, the hendra virus was apparently spread from the fruit bat to
horses, and from horses to a few people who handled the horses.
The deaths and sickness caused by the Nipah and hendra viruses underline
public concerns about the rise of new diseases as well as how
infection-causing microbes have found ways to cross over from surviving in
one species to another. For example, viruses that used to inhabit animals
such as pigs, monkeys or cows, can also now affect human beings. This
"breaking of species barriers" has emerged as a critical part of the chain
of events causing the global rise in infectious diseases. In many cases,
specific viruses, bacteria and other microbes exist only in specific
organisms (plants, marine life, insects, animals or humans). Thus, a
microbe is often "host-specific", surviving only within one or a narrow
range of life-forms, and unable to transfer to or live within other
organisms. But some new diseases have emerged as viruses that used to only
inhabit animals were able to "cross species barriers" and infect humans.
It is widely believed that the crossing of the HIV virus from monkey to
humans led to AIDS whilst the crossing of a virus from an animal (yet to
be identified) to humans is believed to be the source of the Ebola
disease, which first appeared in 1976.
In the case of the so-called "Mad Cow Disease", this infection is believed
to have originated in sheep, then transferred to cows, causing BSE (bovine
spongiform encephalopathy). It is then passed via an infectious agent,
known as a prion protein, through beef consumption to humans to cause CJD
(Creutzfeldt-Jakob Disease). Microbes which used to exist in one or a
narrow range of host organisms are broadening their host range, and when
this includes humans, the result is the emergence of new diseases or new
varieties of existing diseases which are harder to treat. The process is
known as "horizontal gene transfer" or the transfer of gene by infection,
between species that do not interbreed and are unrelated. This has been
known to occur among bacteria and viruses for at least 20 years. It used
to be thought that horizontal gene transfers could not involve humans
because there are genetic barriers between species, according to Dr
Mae-Wan Ho, a scientist at the Open University in the United Kingdom. But
in recent years the full scope of horizontal gene transfer has come to
light. Dr Ho says the evidence shows that transfers occur between very
different bacteria, between fungi, between bacteria and protozoa, between
bacteria and higher plants and animals, between fungi and plants and
between insects.
"There is even a report of a gene that has jumped from fruitflies to
humans where it causes a neurological wasting disease," said Dr Ho. The
genes can be transferred through conjugation (or the mating process);
transduction (transfer with the help of viruses); or transformation
(direct uptake of DNA by the bacteria). Horizontal gene transfers have
occurred in the past but they were relatively rare among multicellular
plants and animals. A new fear by some scientists and ecologists is that
genetic engineering could greatly accelerate gene transfers. In a paper,
Dr Ho and Dr Beatrix Tappeser (from the Institute of Applied Ecology in
Germany), explains that genetic engineering technology is designed to
enable genes to cross species barriers.
"It recombines genetic material in the laboratory between species that
have very little probability of exchanging genes otherwise." Drs Ho and
Tappeser explain it is not easy to transfer genes naturally between
species, as there are cellular mechanisms to excise or inactivate foreign
genes. Genetic engineering is designed to break these natural barriers so
that a gene from one species can be transferred into another. The
technology uses artificially constructed parasitic genetic elements
including viruses as "vectors" to multiply copies of genes and in many
cases to carry and smuggle genes into cells (of the target plant, animal
or human being) which normally exclude them.
Thus, transgenic organisms are made carrying the desired transgenes. The
vector is used to enable the gene to more easily cross the species barrier
successfully. "Vectors are now increasingly engineered to overcome the
cellular defence mechanisms, thus further undermining the ability of the
species' system to resist invasion by exotic genes carried on such
transgenic vectors," said Drs. Ho and Tappeser.
"Many unrelated bacterial pathogens, causing diseases from bubonic plague
to tree blight, are now found to share an entire set of genes for invading
host cells, which have almost certainly spread by horizontal gene
transfer. "There is sufficient evidence that horizontal gene transfer is
responsible for the emergence of both old and new pathogens, and for the
evolution of multiple antibiotic resistance. "We certainly do not need any
more releases of transgenic organisms that would provide yet more vehicles
for horizontal gene transfers." Dr Ho said that there is no evidence (at
least not yet) that genetic engineering has been responsible for the
spread of drug resistance in bacteria, or for creating new pathogens. She
stressed, however, that genetic engineering is inherently a technology
that increases the likelihood of horizontal gene transfer and thus has the
potential of spreading resistance and contributing to new diseases. Of
topical interest to Malaysians (given the Nipah virus) is Dr Hožs concern
about research being conducted to make "transgenic pigs" with organs
suitable for transplanting to humans. The pigs are genetically engineered
so that the pig organs would not be rejected in humans after
transplantation. "This is worrying because the there is a possibility that
pig viruses may then be able to cross species barriers to affect human
beings." Dr Ho said that a laboratory experiment had shown that if pig
viruses were cultured in human cells for only one generation they would
already be able to infect human cells. She called for a moratorium on
environmental releases of transgenic organisms and the marketing of
transgenic foods to be imposed until the possibility of horizontal gene
transfer and its consequences can be fully assessed.
Martin Khor is the Director of Third World Network.
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